The present invention relates to an optical recording medium for recording and reproducing data by using a laser beam and more particularly to an optical recording medium in which an address area is intermittently arranged in a radial direction and the storage capacity of an optical disc is improved by a technique, that is, what is called a zoning method.
To increase a storage capacity by effectively using a recording area in an optical disc, a method called a zoning method has been hitherto employed. This is a method that a reference recording frequency for recording and reproducing data is switched for each radius, for instance, in a disc rotating at a constant angular velocity to have substantially the same recording density throughout all the surface of the disc.
For instance, in ISO/IEC14517 (130 mm 4×), ISO/IEC15286 (130 mm 8×), ISO/IEC15041 (90 mm 5×) or the like as magneto-optical discs of an ISO standard, the storage capacity of the disc is improved by using this method.
That is, a data recording and reproducing area is divided into a plurality of zones in the radial direction as shown in FIG. 1. Since frequencies for recording and reproducing data are respectively different in the zones, the number of sectors in one round changes. Therefore, an address area shows partly radial forms for each radius as shown by 1a. 
Further, in a magneto-optical recording disc, an operation for setting the magnetizing direction of a recording film to a prescribed direction is applied to the disc before the disc is provided for a user. That is, an operation, that is, what is called a polarization is carried out that a static magnetic field larger than the coercive force of a recording layer is applied to the disc to forcedly direct the magnetizing direction of the recording layer to an erasing direction.
Ordinarily, when the recording layer to be polarized has a coercive force as low as 0.8×106 A·m (A·m is SI unit as a magnetic field strength), the recording layer can be polarized with a low magnetic field such as an electromagnet. However, when the recording layer having a coercive force not lower than 1.19×106 to 1.59×106 A·m is polarized, the temperature of the recording layer is raised to lower the coercive force and a low static magnetic field not higher than 0.8×106 A·m is applied to the recording layer to polarize the recording layer.
Further, in a phase-change type disc, an operation for crystallizing all the surface of a recording film is applied to the disc before the disc is provided to a user. That is, the temperature of the recording film is raised to prescribed temperature or higher and the recording film is gradually cooled. Thus, the disc is initialize from what is called an Ad-depo state as a crystalline and amorphous mixed state after the recording film is formed to a completely crystallized state.
In such an initializing process, the temperature of a recording film having a wide area such as several hundred tracks is raised at one time to polarize and crystallize the recording film. This method is called a bulk erase. In the bulk erase method, a semiconductor laser of 1 to 2 watt is restricted to the diameter of an elliptic beam having a major axis of 10 μm or longer in the radial direction of a disc. The rotating disc is irradiated with the laser beam by focusing only on the recording film to raise the temperature of the recording film.
The above-described laser beam is designed, as shown in FIG. 2, to focus an output beam from a laser 5 on a recording film 8 through a resin substrate 7 from the reading surface 6 of the rotating disc.
The above-described bulk erase method has not only a purpose of initializing the recording layer, but also an effect of suppressing a sensitivity change (sensitivity shift) during the use of a medium by a user, by previously taking a recording sensitivity or a reproducing sensitivity generated upon repeated use of the medium. This operation is supposed to be carried out because thermal energy is applied to the recording film to previously relax atoms in an amorphous recording layer and stabilize the recording film.
As parameters of the bulk erase, the rotating speed or the linear velocity of a disc, a radial feed pitch of a laser spot, laser power, the width of a laser beam, etc. are enumerated. These parameters can be easily controlled, so that a polarization or an initialization and a sensitivity shift can be generated in a stable manner by setting optimum bulk erase conditions. Accordingly, the bulk erase method is a very effective method.
When the inventors of the present invention carried out a test of the bulk erase of a magneto-optical recording medium for the purpose of performing the sensitivity shift and the polarization, they found following problems. Specifically, such a bulk erase process as to sufficiently generate a prescribed sensitivity shift is applied to an optical disc zoned in the radial direction and having addresses by emboss pits set in the radial direction respectively in the zones as shown in FIG. 1. Then, they observed tracking errors in the boundaries of the zones. Thus, the existence of an area where the tracking errors were increased was recognized as shown in a signal waveform view in FIG. 3.
FIG. 3 shows a relation between an address signal and a tracking error signal in a position of a tenth track from the zone boundary of the disc whose entire surface undergoes the bulk erase process. (a) designates an address signal, (b) designates a tracking error signal, (c) designates an enlarged part of an area F of the address signal (a), and (d) designated an enlarged part of an area F of the tracking error signal (b), respectively. Further, in the drawing, E designates a track jump signal, G designates an enlarged address in the area F, and H designates a tracking error increasing part.
In measuring conditions in FIG. 3, a linear velocity of 7.5 m/s, CLV, laser power: 1.5 mW, the diameter of a disc of 86 mm, a measuring place R of 40 mm and a land part to which a tracking is applied are included.
As recognized in FIG. 3, the tracking error increasing part H in FIG. 3 is not affected by the address G and has the same positional relation as an address existing in a zone 10 tracks before the tracking error increasing part. The address separated by 10 tracks therefrom gives any effect to the tracking error.
The amount of increase of tracking errors of the tracking error increasing part H reaches as high as 17% relative to the amplitude of the track jump signal E. Further, this phenomenon is observed to spread to several hundred tracks from the zone boundary. Therefore, a drive (disc driving system) recognizes the tracking error increasing areas as defective areas and performs a replacement process. Accordingly, the disc in which an extremely many replaced sectors are present is formed.
When the bulk erase process is not performed, the above-described tracking error increasing phenomenon is not generated. However, when the bulk erase process is not performed, the recording sensitivity changes while a user uses the disc, so that optimum recording and reproducing operations cannot be performed to generate an error.
Further, a method may be proposed that a drive or the like is used to generate the sensitivity shift by a method for recording and erasing each track. However, in this method, the same track needs to be recorded and erased many times. Thus, extremely much time is necessary so that this method is not a realistic method.